Nano- and micro-electromechanical resonators

ABSTRACT

A resonator including a piezoelectric plate and an interdigital electrode is provided. A ratio between a thickness of the plate and a pitch of the interdigital electrode may be from about 0.5 to about 1.5. A radiation detector including a resonator and an absorber layer capable of absorbing at least one of infrared and terahertz radiation is provided. A resonator including a piezoelectric plate and a two-dimensional electrically conductive material is provided.

RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 14/784,876, filed on Oct. 15, 2015, which is a U.S. National Stageof International Application No. PCT/US2014/035015, filed Apr. 22, 2014,which claims the benefit of and priority to U.S. Provisional ApplicationSer. No. 61/814,742, filed Apr. 22, 2013; U.S. Provisional ApplicationSer. No. 61/814,744, filed Apr. 22, 2013; and U.S. ProvisionalApplication Ser. No. 61/828,227, filed May 29, 2013, each of which ishereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No.N66001-12-1-4221, awarded by the DARPA Young Faculty Award and Grant No.DARPA-N66001-14-1-4011 awarded by the DARPA. The United Statesgovernment has certain rights in this invention.

BACKGROUND

The inclusion of resonators in nano-electromechanical systems (NEMS) andmicro-electromechanical systems (MEMS) presents challenges as a resultof the low Figure of Merit of pre-existing resonators. Additionally,pre-existing resonators are not fully scalable as a result of design andmaterial limitations that impose a lower limit on resonator featuresize. These issues preclude the employment of pre-existing resonators inmany applications.

SUMMARY

In view of the foregoing, the present Inventors have recognized andappreciated the advantages of a resonator with the features describedherein to be employed in nano-electromechanical systems (N EMS) and/ormicro-electromechanical systems (MEMS).

Accordingly, provided according to one embodiment herein is a resonatorincluding a piezoelectric plate and an interdigital electrode; a ratiobetween a thickness of the plate and a pitch of the interdigitalelectrode may be from about 0.5 to about 1.5.

In another embodiment, a resonator including a piezoelectric plate andan interdigital electrode is provided; the resonator may be configuredto operate in a combined mode of vibration comprising athickness-extensional mode and a lateral-extensional mode.

In another embodiment, a method of making a resonator is provided. Themethod may include disposing a piezoelectric layer over a substrate,disposing a first electrode layer over the piezoelectric layer,patterning the first electrode layer to form an interdigital electrode,etching the piezoelectric layer to form a piezoelectric micro-plate, andreleasing the micro-plate from the substrate. The resonator may beconfigured to operate in a combined mode of vibration comprising athickness-extensional mode and a lateral-extensional mode.

Provided in another embodiment is a radiation detector including aresonator and an absorber layer. The absorber layer may be capable ofabsorbing at least one of infrared and terahertz radiation.

In another embodiment, a method of making a radiation detector isprovided. The method may include disposing a first electrode over asubstrate, disposing a piezoelectric layer over the substrate and firstelectrode, disposing a second electrode over the piezoelectric layer,disposing an absorber layer over the piezoelectric layer, etching thepiezoelectric layer to form a piezoelectric nano-plate, and releasingthe nano-plate from the substrate.

In another embodiment, a device including a resonator integrated with anabsorber layer capable of absorbing at least one of infrared andterahertz radiation is provided. The device may be configured to carryout at least one of imaging and spectroscopy.

Provided in another embodiment is a resonator including a piezoelectricplate and an electrode comprising a two-dimensional electricallyconductive material.

In another embodiment, a method of making a resonator is provided. Themethod may include disposing a first electrode over a substrate,disposing a piezoelectric layer over the substrate and first electrode,disposing a second electrode comprising a two-dimensional electricallyconductive material over the piezoelectric layer, etching thepiezoelectric layer to form a piezoelectric nano-plate, and releasingthe nano-plate from the substrate.

In another embodiment, a method of operating a resonator is provided.The method may include applying at least one of a direct current biasand a control signal to the resonator. The resonator may include apiezoelectric plate and an electrode comprising a two-dimensionalelectrically conductive material.

It should be appreciated that all combinations of the foregoing conceptsand additional concepts discussed in greater detail below (provided suchconcepts are not mutually inconsistent) are contemplated as being partof the inventive subject matter disclosed herein. In particular, allcombinations of claimed subject matter appearing at the end of thisdisclosure are contemplated as being part of the inventive subjectmatter disclosed herein. It should also be appreciated that terminologyexplicitly employed herein that also may appear in any disclosureincorporated by reference should be accorded a meaning most consistentwith the particular concepts disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The skilled artisan will understand that the drawings primarily are forillustrative purposes and are not intended to limit the scope of theinventive subject matter described herein. The drawings are notnecessarily to scale; in some instances, various aspects of theinventive subject matter disclosed herein may be shown exaggerated orenlarged in the drawings to facilitate an understanding of differentfeatures. In the drawings, like reference characters generally refer tolike features (e.g., functionally similar and/or structurally similarelements).

FIG. 1 depicts an SEM image of an AlN combined mode MEMS resonator,according to one embodiment.

FIGS. 2(a) and 2(b), respectively, are schematic representations ofthickness field excitation and lateral field excitation of contour moderesonators, according to one embodiment.

FIG. 3 depicts the electromechanical coupling coefficient of an AlNlateral field excitation resonator according to one embodiment,employing a single top interdigital electrode as a function of the ratioof the thickness of the AlN resonator to the wavelength.

FIG. 4 depicts the admittance amplitude of a lateral field excitationcombined mode AlN resonator as a function of frequency, according to oneembodiment.

FIGS. 5(a)-(d) are representations of the steps in the manufacture of anAlN combined mode MEMS resonator, according to one embodiment.

FIG. 6 depicts the admittance amplitude of a lateral field excitationcombined mode AlN resonator as a function of frequency, according to oneembodiment.

FIG. 7 depicts a schematic representation of a NEMS resonant infrareddetector, according to one embodiment.

FIG. 8 depicts an SEM image of a NEMS resonant infrared detector,according to one embodiment.

FIG. 9 depicts the admittance amplitude of an AlN resonator without anabsorber layer as a function of frequency, according to one embodiment.

FIG. 10 depicts the admittance amplitude of a NEMS resonant infrareddetector as a function of frequency, according to one embodiment.

FIG. 11 depicts the resonance frequency of a NEMS resonant infrareddetector as a function of temperature, according to one embodiment.

FIG. 12 depicts the temperature of a NEMS resonant infrared detector asa function of time upon exposure to radiation, according to oneembodiment.

FIG. 13 depicts the admittance amplitude of a NEMS resonant infrareddetector as a function of time upon exposure to infrared radiation,according to one embodiment.

FIG. 14 depicts the admittance amplitude of a NEMS resonant infrareddetector as a function of time upon exposure to infrared radiationemitted by a human hand, according to one embodiment.

FIG. 15 depicts the resonance frequency of a NEMS resonant infrareddetector as a function of time upon exposure to broadband infraredradiation, according to one embodiment.

FIGS. 16(a)-16(c), respectively, depict a NEMS resonant infrared sensorconnected to a self-sustained CMOS oscillator circuit for directfrequency readout, Allan Deviation of a NEMS resonant infrared sensoroutput signal as a function of time, and NEMS resonant infrared sensorresonance frequency as a time with an inset depicting NEMS resonantinfrared sensors bonded to a CMOS readout chip, according to oneembodiment.

FIGS. 17(a)-17(e), respectively, depict a schematic representation ofTHz spectroscopy, an integrated wireless resonant imaging array andintegrated CMOS readout, an infrared detector including an AlN resonatora single wall nanotube forest absorber layer, an AlN resonator, and aTHz detector including an MN resonator integrated with an efficientmetamaterial absorber layer, according to one embodiment.

FIGS. 18(a)-18(e) are representations of the steps in the manufacture ofan infrared detector including an AlN resonator and an infrared absorberlayer, according to one embodiment.

FIGS. 19(a)-19(d) are representations of the steps in the manufacture ofa IR/THz detector including an AlN resonator and a IR/THz absorberlayer, according to one embodiment.

FIG. 20 depicts a schematic representation of a graphene containing AlNresonator, according to one embodiment.

FIG. 21 depicts an SEM image of a graphene containing AlN resonator,according to one embodiment.

FIG. 22 depicts the Raman spectrum of the graphene contained in graphenecontaining AlN resonator, according to one embodiment.

FIG. 23 depicts the admittance amplitude of a graphene containing AlNresonator as a function of frequency, according to one embodiment.

FIG. 24 depicts the admittance amplitude as a function of frequency of agraphene containing AlN resonator and a pre-existing AlN resonator thatdoes not contain graphene, according to one embodiment.

FIG. 25 depicts the phase noise as a function of offset frequency of agraphene containing AlN resonator and a pre-existing AlN resonator thatdoes not contain graphene, according to one embodiment.

FIG. 26 depicts the admittance amplitude as a function of frequency of agraphene containing AlN resonator and a fluorinated graphene containingAlN resonator, according to one embodiment.

FIG. 27 depicts the admittance amplitude as a function of frequency of agraphene containing AlN resonator subjected to a variety of biasvoltages, according to one embodiment.

FIG. 28 depicts the impedance variation at resonance as a function of DCbias of a graphene containing AlN resonator and a pre-existing AlNresonator that does not contain graphene, according to one embodiment.

FIGS. 29(a)-29(e) illustrate the steps in the manufacture of a graphenecontaining AlN resonator, according to one embodiment.

FIG. 30 depicts an oscillator circuit containing a graphene containingAlN resonator, according to one embodiment.

FIGS. 31(a) and 31(b) are schematic representations of nano-plateresonators including a 2D electrically conductive electrode in aconductive state and a non-conductive state, respectively, according toone embodiment.

DETAILED DESCRIPTION

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, micro-electromechanical resonators andmethods of producing the same. It should be appreciated that variousconcepts introduced above and discussed in greater detail below may beimplemented in any of numerous ways, as the disclosed concepts are notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes.

Aluminum Nitride Resonator

Aluminum nitride (AlN) contour mode resonator (CMR) high frequencymicro-electro-mechanical systems (MEMS) may be valuable for theapplications in the fields of radio frequency (RF) communication andphysical and chemical sensing at least as a result of high qualityfactor, low motional resistance, complementary metal-oxide-semiconductor(CMOS) compatibility and desirable scaling capabilities. Theelectromechanical coupling coefficient (k_(t) ²), which according to oneembodiment is a measure of the conversion efficiency between electricaland acoustic energy in piezoelectric materials, of an AlN contour-modeMEMS resonators may be relatively low: according to one embodimenttypically lower than 1.5% for single interdigital electrode excited(lateral field excitation (LFE)) high frequency contour-extensional modeAlN MEMS resonators. According to one embodiment, the CMR technologyexhibits the same advantages of thin film bulk acoustic resonators(FBARs) over SAW devices in terms of miniaturization and IC integrationcapabilities. Nevertheless, the CMR technology employs the d₃₁piezoelectric coefficient of AlN to transduce a lateral-extensional modeof vibration, in contrast to FBARs, which use the d₃₃ piezoelectriccoefficient of AlN to transduce a thickness-extensional mode in themicromechanical structure. Therefore, according to one embodiment theoperating frequency of CMRs may be lithographically determined by thelateral dimensions of the device (rather than by the thickness of theAlN layer as in the FBAR case), enabling the fabrication of multiplefrequencies of operation on the same silicon chip. This is an importantfeature for advanced wireless communication systems, for which at leastin one instance single-chip, multi-band RF solutions are becoming thedominant trend.

The Figure of Merit of a resonator according to one embodiment isdefined as the product of the quality factor (Q) and k_(t) ². The Figureof Merit may directly determine the motional resistance in anyresonator, impact oscillator design by setting the required gain (i.e.power consumption) and phase noise of oscillator, and impact filterdesign by setting insertion loss in properly terminated filters anddevice bandwidth.

The resonators may be part of nano-electromechanical ormicro-electromechanical systems. According to one embodiment, theresonator may be a plate type resonator. The plate may have any suitabledimensions. Depending on the geometry of the resonator, the term“dimension” may refer to any dimension of interest in the resonator. Forexample, the dimension may refer to thickness, width, height, length,diameter, radius, etc. According to one embodiment, the dimension mayrefer to the thickness of the plate. According to one embodiment, theplate may have a thickness of less than or equal to about 10microns—e.g., less than about 9 microns, about 8 microns, about 7microns, about 6 microns, about 5 microns, about 4 microns, about 3microns, about 2 microns, about 1 micron, about 900 nm, about 800 nm,about 700 nm, about 600 nm, about 500 nm, about 400 nm, about 300 nm,about 200 nm, about 100 nm, or less. According to another embodiment,the plate is a nano-plate, referring to a plate whose largest dimensionis in the nanometer range, such as less than or equal to about 1 micron,or any of the aforedescribed values with respect to dimension. Accordingto another embodiment, nano-plate may refer to a plate with at least onedimension in the nanometer range, such as less than or equal to about 1micron, or any of the aforedescribed values with respect to dimension.According to another embodiment, the plate is a micro-plate, referringto a plate whose largest dimension is in the micrometer range, such asless than or equal to about 1 mm. According to another embodiment,micro-plate may refer to a plate with at least one dimension in themicrometer range, such as less than or equal to about 1 mm.

The plate may include any suitable piezoelectric material. According toone embodiment, the plate may include a compound, such as a nitride,such as an aluminum nitride (AlN). According to another embodiment, theplate a may include at least one of aluminum nitride, lithium niobate,lithium tantalate, zinc oxide, gallium nitride and quartz. Theelectrodes of the resonator may include any suitable material. Accordingto one embodiment, the electrodes may include a metal, such as a noblemetal, such as platinum or gold.

Combined Mode of Vibration Resonator

The performance of an electromechanical resonator is in generalevaluated in terms of Figure of Merit (FOM), defined as the product ofthe quality factor (Q) and the electromechanical coupling coefficient(k_(t) ²) of the resonator. A high Q>2000 may generally be achieved inhigh frequency AlN CMRs, while only limited values of k_(t) ² have beendemonstrated for this type of device due at least in part to therelatively small value of the d₃₁ piezoelectric coefficient of AlNemployed to excite the contour-extensional mode of vibration. Bycontrast, FBAR devices may exhibit higher values of k_(t) ²˜6% at leastbecause the d₃₃ piezoelectric coefficient may be more than twice aslarge as the d₃₁ coefficient. Furthermore, such piezoelectriccoefficient-limited values of k_(t) ² may be achieved by maximizing theconfinement of the excitation electric field across the thickness of theAlN layer, through the use of a thickness field excitation (TFE) schemeinvolving a top and a bottom electrodes produced utilizing a 4-maskfabrication process. FIG. 2(a) depicts a TFE scheme according to oneembodiment, in which positive electrodes 30 and negative electrodes 40are present on either side of a piezoelectric plate 50, such that anelectric field 10 is established between the electrodes, producing anin-plane displacement 20 of the piezoelectric material.

Lateral field excitation (LFE) schemes allow the use of a singleinterdigital electrode on the AlN plate. Such an LFE configuration maysimplify the fabrication of the device by utilizing a 2-mask fabricationprocess. FIG. 2(b) depicts an LFE scheme in which positive electrodes 30and negative electrodes 40 are present on one side of a piezoelectricplate 50 such that an electric field 10 is established between theelectrodes producing an in-plane displacement 20 of the piezoelectricmaterial. Furthermore, operation in the super-high frequency band (e.g.,up to about 9.9 GHz) and values of Q as high as about 2200 have beendemonstrated using LFE. Pre-existing LFE schemes may produce relativelylow values of k_(t) ²<1.5%, as a result of the reduced confinement ofthe excitation electric field across the thickness of the AlN layer.

A pre-existing LFE CMR is composed of a simple two-layer structure, inwhich an interdigital metal electrode is deposited on an AlN plate. Whenan AC signal is applied to the top interdigital electrode, acontour-extensional mode of vibration may be excited through theequivalent d₃₁ piezoelectric coefficient of AlN. Given the equivalentmass density, ρ_(eq), and Young's modulus, E_(eq), of the material stackthat forms the resonator, the center frequency, f₀, of this laterallyvibrating mechanical structure, may be determined by the pitch, W, ofthe interdigital electrode.

A combined mode of vibration resonator may allow a high operatingfrequency (˜2.8 GHz), quality factor (Q˜4855) and electromechanicalcoupling coefficient (k_(t) ²˜2.48%) to be simultaneously demonstratedin an LFE AlN MEMS resonator. The thickness and lateral dimensions ofthe resonator may be designed to transduce a single mechanical combinedmode of vibration based on the coherent combination of the d₃₁ and thed₃₃ piezoelectric coefficients of AlN. Such a combination ofpiezoelectric coefficients produces an enhanced k_(t) ² of the deviceresulting in a 2.8 GHz LFE AlN MEMS resonator with unprecedentedly highFOM˜46 with the use of a single interdigital top-electrode producedutilizing a simple 2-mask fabrication process. The pitch, W, of theinterdigital electrode and a thickness, T, of the AlN layer of thecombined mode resonator may be selected in order to excite a singlemechanical mode of vibration based on the coherent combination of d₃₁and d₃₃ piezoelectric coefficients to maximize the electromechanicalcoupling coefficient of the device.

The k_(t) ² dependence on the thickness, T, of the AlN resonator for agiven value of the interdigital electrode pitch, W, hence wavelength, λ,as λ=2 W was investigated. As shown in FIG. 3, high values of k_(t)²>2.5% (˜twice the best values achievable with pre-existing LFE AlNCMRs) may be achieved for T/λ values ranging between about 0.35 andabout 0.55 for which d₃₁ and the d₃₃ piezoelectric coefficients arecoherently exploited to excite a single combined lateral-thicknessextensional mode of vibration. The data indicate that theelectromechanical coupling coefficient of combined mode resonators mayreach about 3%. The admittance curve and the corresponding 2D mode shapeof vibration for T/A=0.47 are shown in FIG. 4. The result shows that, atthe resonance frequency, the lateral-extensional andthickness-extensional modes are combined into a single mechanical modeof vibration with an enhanced electromechanical coupling coefficient,k_(t) ² 2.78%. A spurious vibration mode at a frequency slightly lowerthan the resonance frequency may also be observed.

The resonator may have any appropriate plate thickness to interdigitalelectrode pitch ratio (T/W) for a combined mode of vibration. Accordingto one embodiment, the resonator exhibits a thickness to interdigitalelectrode pitch ratio of about 0.5 to about 1.5—e.g., about 0.6 to about1.4, about 0.7 to about 1.3, about 0.8 to about 1.2, about 0.9 to about1.1, etc. According to another embodiment, the resonator may beconfigured to operate in a combined mode of vibration, such as acombined mode of vibration in which at least both the d₃₁ and d₃₃piezoelectric coefficients contribute to the electromechanical couplingcoefficient of the resonator.

The resonator may have any appropriate electromechanical couplingcoefficient. According to one embodiment, the resonator exhibits anelectromechanical coupling coefficient of at least about 1.5%—e.g., atleast about 1.6%, about 1.7%, about 1.8%, about 1.9%, about 2%, about2.1%, about 2.2%, about 2.3%, about 2.4%, about 2.5%, or more.

The resonator may have any appropriate quality factor. According to oneembodiment, the resonator exhibits a quality factor of at least about1600—e.g., at least about 1700, about 1800, about 1900, about 2000,about 2100, about 2200, or more.

The resonator may be configured to resonate at any appropriatefrequency. According to one embodiment, the resonator resonates at afrequency of at least about 10 MHz—e.g., at least about 50 MHz, about100 MHz, about 200 MHz, about 300 MHz, about 400 MHz, about 500 MHz,about 600 MHz, about 700 MHz, about 800 MHz, about 900 MHz, or higher.According to another embodiment, the resonator resonates at a frequencyof about 10 MHz to about 100 GHz—e.g., about 50 MHz to about 90 GHz,about 100 MHz to about 80 GHz, about 200 MHz to about 70 GHz, about 300MHz to about 60 GHz, about 400 MHz to about 50 GHz, etc.

Not to be bound by any theory, but the combined vibration mode mayimprove the Figure of Merit of AlN MEMS resonators by increasing theelectromechanical coupling coefficient (k_(t) ²) while preserving a highquality factor (Q), thereby enabling the implementation of highperformance, miniaturized and high frequency AlN MEMS resonators. Thecombined vibration mode may produce a device with a Figure of Merit ofat least about 40—e.g., at least about 45, about 50, about 55, about 60,or higher. The high Figure of Merit of the combined mode of vibrationAlN MEMS resonators allows the implementation of low motional resistanceMEMS resonators, high gain (low power consumption) self-sustained andcompact MEMS-CMOS oscillators and low insertion loss MEMS filters. Thehigh Figure of Merit allows the device motional resistance (R_(m)) to besignificantly reduced. According to one embodiment, the resonatorexhibits a motional resistance of about 15Ω, which is much smaller thanthat of pre-existing AlN contour-mode resonators working in the GHzrange that exhibit typical motional resistances of ˜100s Ω. According toone embodiment, the resonator may exhibit a motional resistance of lessthan or equal to about 100Ω—e.g., less than or equal to about 90Ω, about80Ω, about 70Ω, about 60Ω, about 50Ω, about 40Ω, about 30Ω, about 20Ω,about 15Ω, or less. Pre-existing AlN contour-mode resonators working inthe GHz range exhibit Figures of Merit below about 30. The improvedFigure of Merit of the resonators described herein, as compared with thepre-existing technology, may allow the development of high performance(low power consumption, low insertion loss and compact) devices, such asMEMS-CMOS oscillators and band-pass filters, based on the combined modeAlN MEMS resonators.

The resonators described herein may be fabricated by any appropriateprocess. According to one embodiment, the resonators may be fabricatedby a simplified two-mask microfabrication process. The fabricationprocess may include: disposing a piezoelectric layer over a substrate,disposing a first electrode layer over the piezoelectric layer,patterning the first electrode layer to form an interdigital electrode,etching the piezoelectric layer to form a piezoelectric micro-plate, andreleasing the micro-plate from the substrate. The substrate maycomprise, or be, any suitable material, such as silicon. According toone embodiment, the substrate may be a silicon wafer. According to oneembodiment, the disposing of the piezoelectric layer may include anysuitable process, such as a sputter deposition process. Thepiezoelectric layer may include any suitable material, such as theaforedescribed piezoelectric materials. According to one embodiment, thedisposing of the first electrode layer may include any suitable process,such as a sputter deposition process. According to one embodiment, thepatterning of the first electrode layer to form an interdigitalelectrode may include any suitable process, such as a lift-off process.According to one embodiment, the etching of the piezoelectric layer toform a piezoelectric micro-plate may include any suitable process, suchas an ion conductive plasma (ICP) process. The forming of themicro-plate may include forming a perimeter of the nano-plate. Accordingto one embodiment, the releasing the piezoelectric layer from thesubstrate may include any suitable process, such as an isotropic etchingprocess.

An AlN resonator with T/λ=0.47 was designed and fabricated. The pitch120, W, of the interdigital electrode was set to 1.6 μm, and thethickness, T, of the AlN was accordingly chosen to be 1.5 μm, producingt/W≈0.94. The AlN resonator was fabricated by a simplified two-maskmicrofabrication process. As depicted in FIG. 5(a), a 1.5 μm thick MNfilm 150 was sputter-deposited on top of a high resistivity siliconwafer substrate 130. Then, a 200/50 nm thick Al/TiN film was depositedon top of the AlN film and patterned by a lift-off process to producethe interdigital electrode 110 as shown in FIG. 5(b). Next, the AlN filmwas etched by ICP in Cl₂ based chemistry to define the perimeter of theresonator by producing gaps 160 in the AlN film as shown in FIG. 5(c).Finally, the silicon substrate underneath the AlN resonator was releasedby XeF₂ isotropic etching producing a release void 165 in the siliconsubstrate 150 as shown in FIG. 5(d). An SEM image of the fabricatedresonator 100 is shown in FIG. 1.

The electrical performance of the fabricated AlN LFE combined mode MEMSresonator was measured by an Agilent E5071C network analyzer afterperforming an open-short-load calibration. The measured admittanceresponse versus frequency and the Butterworth-van Dyke (BVD) fitting areshown in FIG. 6. A single resonance peak was measured at 2.82 GHz. Ahigh quality factor value, Q of 1855 as a result of the directdeposition of AlN film on a polished silicon substrate and highelectromechanical coupling coefficient, k_(t) ²˜2.48%, as a result ofthe combined d₃₁ and d₃₃ piezoelectric coefficients, was achieved. Themeasured k_(t) ² of 2.48% and the corresponding FOM˜46 are the highestever reported among LFE AlN MEMS resonators employing a single topinterdigital electrode produced utilizing a 2-mask fabrication process.A motional resistance of about 15Ω was measured.

Radiation Detector Including Resonator

Infrared (IR) detectors may be employed in military and spaceapplications, such as night vision, surveillance and targeting andcivilian applications, such as health care, automotive, chemical andbiological sensing, and telecommunications. IR sensors may be generallydivided in two categories: photonic detectors and thermal detectors.Semiconductor photonic detectors exhibit spectrum selectivity, lowdetection limit and fast response. However, to achieve such performance,cryogenic cooling, which is expensive, bulky, and power consuming, isgenerally needed in pre-existing processes to prevent the formation ofthermally generated carriers. On the other hand, thermal detectors,which have been implemented by bolometers, pyroelectric IR detectors andthermopiles, are less expensive, compact and more power efficient.However, pre-existing thermal detectors cannot rival semiconductorphotonic IR detectors in terms of detection limit and response time.

With the recent advances in Micro/Nano-Electro-Mechanical Systems(MEMS/NEMS), uncooled IR thermal detectors based on MEMS/NEMS technologyhave attracted much attention due to their potentially ultra-highresolution and advantages in terms of size and cost, compared topre-existing cryogenically cooled semiconductor photon detectors.Micromachined uncooled resonant IR thermal detectors based on galliumnitride and Y-cut-quartz demonstrate promising IR detectioncapabilities. An ultra-fast, sensitive and miniaturized uncooled NEMSthermal imaging systems is provided based on a high frequency (196.6MHz) AlN nanomechanical resonant structure. Different from quartz andgallium nitride, ultra-thin (10s nm thick) and high quality AlNpiezoelectric films can be deposited by low-temperature sputteringprocesses directly on silicon substrates, enabling the implementation ofCMOS compatible, ultra-miniaturized and high frequency resonantstructures with desirable thermal detection capabilities. The scalingcapabilities and the desirable transduction properties of AlN atnanoscale allow for the first demonstration of a fast and highresolution IR resonant thermal detector based on a 250 nm thick AlNnano-plate. An ultra-thin Si₃N₄ film (100 nm thick) was employed as anIR absorber, resulting in a NEMS resonant IR detector whose resonancefrequency is highly sensitive to IR radiation in the 8 to 14 μm spectralrange, making it suitable for human tracking applications. The NPR maybe configured to be excited in a contour-mode of vibration bypiezoelectric transduction, as described above.

The integrated absorber layer on the AlN NPR may be any appropriatematerial. According to one embodiment, the absorber layer may include asingle wall carbon nanotube (SWNT) forest, silicon nitride, graphene ora metamaterial. The metamaterial may be a photonic metamaterial, such asa photonic metamaterial comprising patterned metal or graphene. Theselection of the absorber layer may allow the radiation detector todetect infrared radiation, terahertz radiation, or both. According toone embodiment, the absorber layer may also act as a second electrode ofthe nano-plate resonator.

A schematic representation of the AlN NPR NEMS IR detector is shown inFIG. 7. The IR detector includes an AlN nano-plate resonator 200 workingat a higher order contour-extensional mode of vibrations viapiezoelectric transduction. An interdigital electrode 210 on the bottomof the AlN layer may excite the high frequency bulk acoustic mode ofvibration into the nano-plate 250 while an electrically floating thinmetal plate 212 (100 nm thick gold) on top of the AlN nano-plate mayconfine the electric field within the AlN layer and improve theelectromechanical coupling. Probing pads 214 may be electricallyconnected to the interdigital electrode 210 for the purposes ofconnecting the resonator to electrical circuits. A thin Si₃N₄ layer 270(100 nm thick) may be integrated on top of the gold electricallyfloating electrode as an effective LWIR absorber. The thickness of suchSi₃N₄ layer 270 directly affects both the IR absorptivity (the thickeris the Si₃N₄ layer, the higher is the absorption) and the qualityfactor, Q, (the thinner is the Si₃N₄ layer, the lower the mechanicalloading effect on the resonator and the higher the Q) of the device. Atradeoff between absorptivity and quality factor may be considered tooptimize the thickness of the integrated Si₃N₄ absorber. The thicknessof the Si₃N₄ absorber was set to 100 nm in order to avoid significantmechanical loading and Q degradation of the ultra-thin (250 nm thick)NPR. The thickness of the Si₃N₄ layer produces absorptivity as high as15% in the LWIR range.

The resonance frequency of the AlN NPR IR detector can be expressed as

${f = {\frac{1}{2\; W_{0}}\sqrt{\frac{E_{eq}}{\rho_{eq}}}}},$where, W₀ is the pitch of the interdigital bottom electrode, and E_(eq)and ρ_(eq) are respectively the equivalent Young's modulus and densityof the material stack forming the resonator. Because of the highlytemperature-dependent equivalent Young's modulus, the resonancefrequency of the device is highly sensitive to temperature, with typicalvalues of the temperature coefficient of frequency, TCF, on the order ofabout −30 to about −40 ppm/K, with values of down to about −100 ppm/K.According to one embodiment, the TCF may be less than about −30 ppm/K,or less. According to one embodiment, the desirable thermal isolation(ultra-high thermal resistance) and low thermal mass of the nano-plateresonator (i.e. ˜500 nm thick freestanding membrane) result in theabsorbed IR radiation producing a large and fast increase of the devicetemperature. According to one embodiment, the temperature increasetranslates to a large and fast variation of the device mechanicalresonance frequency due to the high TCF of the device. The high qualityfactor, Q, of the resonant system, which is not exhibited bypre-existing microbolometers, allows the detection of smallIR-radiation-induced frequency variations, resulting in a fast andhigh-resolution IR detector. For this kind of resonant IR detectordescribed herein, a lower limit of detection and faster response may beachieved by scaling the volume of the NEMS resonator to reduce thethermal mass and increase the thermal resistance while maintaining highQ values, to maintain low noise performance. The desirable scalingcapabilities and transduction properties at nanoscale of AlNultra-thin-films allow the thickness of the AlN nano-plate resonator tobe about 250 nm, which is thinner than the previously demonstrated MEMSresonant IR detectors implemented with gallium nitride (2.15 μm) andquartz (6.9 μm). Other thickness values are also possible in theresonators provided herein, as described above.

The NPR radiation detector may be fabricated by any suitable fabricationprocess. According to one embodiment, the radiation detector may befabricated by a five-mask post-CMOS compatible microfabrication process.The fabrication process may include: disposing a first electrode over asubstrate, disposing a piezoelectric layer over the substrate and firstelectrode, disposing a second electrode over the piezoelectric layer,disposing an absorber layer over the piezoelectric layer, etching thepiezoelectric layer to form a piezoelectric nano-plate and releasing thenano-plate from the substrate. The substrate may be any suitablematerial, such as silicon. According to one embodiment the substrate maybe a silicon wafer. According to one embodiment, the disposing of thepiezoelectric layer may include any suitable process, such as a sputterdeposition process. The piezoelectric layer may include any suitablematerial, such as the aforedescribed piezoelectric materials. Accordingto one embodiment, the disposing of the first electrode may include anyappropriate process, such as a sputter deposition process. The firstelectrode may be an interdigital electrode. According to one embodiment,the disposing of the second electrode may include any appropriateprocess, such as a sputter deposition process. According to oneembodiment, the disposing of the absorber layer may include anyappropriate process, such as a chemical vapor deposition (CVD) process.According to one embodiment, the etching of the piezoelectric layer toform a piezoelectric nano-plate may include any suitable process, suchas an ion conductive plasma (ICP) process. The forming of the nano-platemay include forming a perimeter of the nano-plate. According to oneembodiment, the releasing the piezoelectric layer from the substrate mayinclude any suitable process, such as an isotropic etching process.

An exemplary AlN NPR IR detector was fabricated by a five-mask post-CMOScompatible microfabrication process. A high resistivity Silicon (Si)wafer (>10000 Ω·cm) was used as a substrate. A 50 nm thick Platinum (Pt)film was sputter-deposited and patterned by lift-off on top of the Sisubstrate to define a bottom interdigital electrode. Then, a 250 nm AlNfilm was sputter-deposited, and vias to access the bottom IDT electrodewere etched by H₃PO₄. Next, a 100 nm thick Au film was deposited andpatterned by lift-off as a top floating metal plate to confine theelectrical field. Then, a 100 nm thick Si₃N₄ film was deposited byplasma enhanced chemical vapor deposition (PECVD) and patterned on topof the Au plate as an IR absorber. The AlN film was etched byInductively Coupled Plasma (ICP) etching in Cl₂ based chemistry todefine the perimeter of the resonant nano-plate. Finally, the structurewas released from the substrate by XeF₂ isotropic etching of the siliconsubstrate. An SEM image of a fabricated AlN NPR IR detector 200 is shownin FIG. 8, including supports 252 and gaps 260 that allow the movementof the resonator nano-plate. The device shown in FIG. 8 has a length, L,of 200 μm, a width, W, of 100 μm and a pitch, W₀, of 11 μm, producing aresonance frequency of 196.6 MHz.

To calibrate the IR detector, a reference device based on the same coredesign of the AlN NPR IR detector but without a Si₃N₄ thin film absorberwas fabricated on the same wafer. The electrical response of thefabricated reference device and IR detector were measured by an AgilentE5071C network analyzer after performing a short-open-load calibrationon a reference substrate. The measured admittance and Butterworth-VanDyke (BVD) fitting curves versus frequency of the reference device andthe AlN NPR IR detector are shown in FIGS. 9 and 10. respectively. Theresonance frequency of the reference device and the IR detector weremeasured to be 233.3 MHz and 196.6 MHz, respectively. The relativelylower resonance frequency of the IR detector is a result of the loadingeffect of the Si₃N₄ film on top of the AlN NPR. However, despite theintegration of the Si₃N₄ absorber, a high device Figure of Merit (17.2)was achieved compared with the reference device (20.5). Despite thereduced volume of the fabricated AlN NPR IR detector, a high qualityfactor (Q˜1062) was achieved comparable to quality factors previouslydemonstrated by ˜10λ larger volume gallium nitride MEMS IR detectors(Q˜1200) and ˜200× larger volume quartz MEMS IR detectors (Q˜4200),demonstrating the desirable scaling advantages of the AlN NPR IRdetector provided herein.

The IR response of the fabricated NEMS IR detector was tested bymeasuring the admittance amplitude change of the device when exposed toIR radiation generated by a Cool Red light source from Ocean Optics witha wavelength of 0.9-25 μm coupled to an optical fiber with atransmission range of 300 nm-4.5 μm. The transient response of thedevice was measured by exciting the resonator at a single frequency,f_(c), and monitoring the variation over time of the device admittanceamplitude. The excitation frequency, f_(c)=196.97 MHz, was set betweenthe series and parallel resonances, where the slope of admittanceamplitude curve versus frequency is at a maximum (0.0263 dB/kHz). Theresponsivity of the IR detector was calculated by multiplying the slopeof the admittance amplitude versus frequency (0.0263 dB/kHz) by the TCF(−30 ppm/K) and the thermal resistance (56 mK/˜W), producing aresponsivity of ˜8837 dB/W. The TCF of the NEMS IR detector wasdetermined to be about −30 ppm/k based on the measurement of theresonance frequency as a function of temperature, as shown in FIG. 11.The inset of FIG. 11 shows the resonant frequency of the referencedevice as a function of temperature, producing a TCF of the referencedevice of about −34 ppm/K. The thermal time constant of the NEMS IRdetector was determined on the basis of temperature response of thedetector as a function of time under exposure to IR radiation to beabout 1.4 ms, as shown in FIG. 12. The inset of FIG. 12 depicts theuniform temperature distribution across the resonant detector. Themeasured response of the IR detector is shown in FIG. 13, with the insetshowing that no response was recorded for the reference device exposedto IR radiation under the same conditions. An admittance amplitudevariation of 0.002 dB was recorded and, given the sensor responsivity of˜8837 dB/W, the amount of IR power absorbed by the device was extractedto be ˜226 nW. The root mean square (RMS) noise of the measuredadmittance amplitude in 1 Hz bandwidth was extracted to be 21.4μdB/Hz^(1/2). The noise equivalent power (NEP) of the IR detector wasestimated by dividing the measured noise spectral density (RMS noise in1 Hz bandwidth measurement) by the responsivity, and was found to be 2.4nW/Hz^(1/2).

Since the radiation peak of the human body is around ˜10 μm, the use ofSi₃N₄ thin film absorber (wavelength absorption range of 8-14 μm) mayallow the device to be capable of human body tracking applications. Toinvestigate the capability of the NEMS IR detector to detect human bodyradiation, a human hand was placed in the field of view of the deviceand its response was monitored over time, as shown in FIG. 14. A maximumadmittance amplitude change of 0.48 dB was recorded when a human handwas in the field of view of the IR detector (˜1 cm away from thedevice), which corresponds to a device temperature rise of ˜3 K, and ˜54μW of IR power absorbed by the detector (given the thermal resistance of56 mK/μW estimated by FEM simulation). The perturbation of the deviceresponse is due to small movements of the imaged human hand, indicatingthe high sensitivity and the fast response time of the device.

Despite the overall reduced volume and the integration of the Si₃N₄absorber, the device showed high electromechanical performance (qualityfactor Q=1062 and electromechanical coupling coefficient k_(t) ²=1.62%).By taking advantage of the aforedescribed desirable scaling andtransduction properties of AlN at nanoscale, a high resolution (measuredNEP˜2.4 nW/Hz^(1/2)) and fast (thermal time constant˜1.4 ms) IR detectorprototype was experimentally demonstrated indicating the great potentialof this technology for the implementation of ultra-fast and sensitivethermal imaging systems.

The IR response of the system was tested with a frequency counter and abroadband IR source with a wavelength range of 900 nm to 25 μm. Themeasured IR response of the system is shown in FIG. 15. The IR sourcewas placed 10 cm away from the IR detector chip and the gate time of thefrequency counter was set to be 100 ms. A thermal time constant of 1.3ms and a measured responsitivity of 267 Hz/μW were achieved. Theextracted noise equivalent power (NEP) of the device was 18 nW/Hz^(1/2).

Compact and low-power CMOS circuits may be integrated with the radiationdetector as direct frequency readouts, due to the high electromechanicalperformance of the resonant structure, as shown in FIG. 16(a). Accordingto one embodiment, the CMOS circuits may be configured to provide anelectronic readout of the frequency of the nano-plate. According toanother embodiment, any appropriate electronic circuit may be integratedwith the radiation detector to provide an electronic readout of thefrequency of the nano-plate. An AlN NPR chip bonded to a CMOS readoutchip fabricated in an AMIS 0.5 μm CMOS process is shown in the inset ofFIG. 16(c). FIG. 16(b) depicts the measured Allan Deviation of thefabricated sensor output signal, showing an ultra-low noise inducedfrequency fluctuation of only about 3.5 Hz at a resonant frequency of122.6 MHz, or 28 ppb.

The most diffused pre-existing technique used to perform IR/THzspectroscopy Fourier Transform Infrared (FTIR) suffers from fundamentallimitations that may prevent the implementation of compact, lightweightand portable field-based analytical tools for rapid and reliableidentification of unknown hazardous gases and vapors for homelandsecurity, defense, intelligence and healthcare applications. Thechallenge in bringing FTIR spectroscopy out of the laboratory and intothe field may be attributed to that the maximum attainable spectralresolution (in cm units) of a FTIR system is inversely related to themaximum retardation (in length units) produced by the interferometer. Asa result, high-resolution FTIR spectrometers may need high retardationand, thus, a large amount of travel in the moving mirror of theinterferometer. For example, resolving spectral features with 100 MHzresolution would need a mirror travel of 3 m. This limitation rendersFTIR inadequate for the implementation of miniaturized andhigh-resolution field-based spectrometers.

A desirable frequency-domain IR spectroscopy system may pass broad bandIR radiation directly through the sample of interest and focus theradiation onto a multi-color IR/THz detector array capable of directmeasurement of the frequency-dependent transmission through the sampleof interest. Such a frequency-domain technique may have severaladvantages compared to FTIR, including but not limited to: it does notneed mechanical moving mirrors and produces higher spectral resolutionby eliminating limits associated with the travel of the moving mirror,it offers the possibility to selectivity scan specific frequency regionsof interest with adjustable resolution, and it produces a fasteracquisition time since transmission data at different frequencies issimultaneously acquired and no sampling is required. Despite theextraordinary potential of ideal frequency-domain IR/THz spectroscopysystems, the actual implementation of compact, low cost and highperformance frequency domain IR spectrometers has previously beenprevented by the fundamental lack of high resolution, fast, highlyminiaturized, uncooled and multi-color IR detector technology capable ofdetecting low power levels.

The ultra-high resolution, fast and multi-color IR NEMS/MEMS resonantdetectors operating at room temperature and without active coolingdisclosed herein may enable the implementation of a new generation ofminiaturized and efficient frequency-domain IR spectroscopy systems. Aschematic representation of how such a spectroscopy system would operateaccording to one embodiment is depicted in FIG. 17(a). These systemshave applications including defense and intelligence applications suchas chemical and biological threat detection, night vision, infraredhoming, multi-color infrared seeking, remote sensing, distributedenvironmental monitoring and healthcare applications such as pathogenand drug detection and identification.

As shown in FIG. 17(c), the IR/THz detector includes two components: thecore element is an AlN nano-plate (10s of nm thick) resonator 250efficiently excited in a contour-mode of vibration by piezoelectrictransduction as shown in FIG. 17(d), and the other is an integratedabsorber on top of the AlN NPR, which may include any IR/THz radiationabsorbing materials/structures. As shown in FIG. 17(b), compact andlow-power CMOS circuits may be directly integrated with the radiationdetector as direct frequency readouts 202, due to the highelectromechanical performance of the resonant structure. The resonator200 integrated with the absorber layer is depicted in FIG. 17(e).

According to one embodiment, an infrared detector based on a nano-plateresonator and an integrated absorber may be fabricated. The absorber maybe an Si₃N₄ absorber. An exemplary fabrication process is shown in FIG.18(a), a Pt bottom electrode 210 was formed on a high resistance siliconsubstrate 230 by a sputter deposition and lift-off process. An AlN layerwas then disposed over the silicon substrate 230 and the bottomelectrode 210 by a sputter deposition process, as shown in FIG. 18(b).The AlN layer was wet etched with H₃PO₄ to expose a portion of thebottom electrode. As shown in FIG. 18(c), a layer of Si₃N₄ 270 wasdeposited by a plasma enhanced chemical vapor deposition process (PECVD)followed by patterning by an ICP process. Probing pads 214 including Ptor Au were formed by a sputter deposition and lift-off process. The AlNwas then dry etched in Cl₂ based chemistry to form gaps that defined theperimeter of the nano-plate, as shown in FIG. 18(d). The nano-plate wasthen released from the silicon substrate and a release void 265 wasformed via an XeF₂ dry release process.

According to another embodiment, an infrared/terahertz detector based ona nano-plate resonator and an integrated metamaterial absorber may befabricated. The absorber may be a Pt metamaterial. The detector mayinclude an air gap between the nano-plate resonator and the absorber.According to one embodiment, the air gap may be formed by a separationbetween the nano-plate resonator and the absorber of at least about 1nm. According to another embodiment, the air gap may be formed by aseparation between the nano-plate resonator and the absorber of lessthan about 1 mm. A process for fabricating a detector including an airgap may additionally include the steps of: disposing a sacrificial layerover the piezoelectric layer, disposing the absorber layer over thesacrificial layer and removing the sacrificial layer to form an air gapbetween the absorber and the piezoelectric layer. The sacrificial layermay be any suitable material, such as polysilicon. The disposing of thesacrificial layer may include any suitable process. The removing of thesacrificial layer may include any suitable process, such as the processemployed to release the nano-plate from the substrate. An exemplaryfabrication process included forming a Pt bottom electrode 212, AlNlayer 230 and Al top electrode 210 on a silicon substrate 250 by asputter deposition and lift off process, as shown in FIG. 19(a). The AlNwas then dry etched in Cl₂ based chemistry to form gaps that defined theperimeter of the nano-plate, as shown in FIG. 19(b). A polysiliconsacrificial layer 280 was then deposited and patterned over thestructure, followed by deposition and patterning of a silica nano-plate282 and deposition and patterning of a Pt metamaterial, as shown in FIG.19(c). A dry release XeF₂ process was employed to release the nano-plateresonator from the substrate, form a release void 265 and to remove thesacrificial polysilicon layer to form an air gap 290 between theabsorber and the resonator, as shown in FIG. 19(d).

Resonator Including 2D Electrically Conductive Material

Micro- and Nano-electromechanical systems (MEMS/NEMS) resonators may bevaluable for multiple sensing applications at least as a result of thecombination of high sensitivity to external perturbations due to theirvery reduced dimensions, and ultra-low noise performance due to theintrinsically high quality factor, Q, of such resonant systems. Amongdifferent MEMS/NEMS resonant sensors, the AlN nano plate resonant sensor(NPR-S) technology, which involves exciting high frequency (about 100MHz to about 10 GHz) bulk acoustic waves in piezoelectric nano-plates(thickness <1 μm) comprising AlN may be a desirable solution for therealization of sensitive, miniaturized and low power chemical sensors,thermal detectors, and magnetic field sensors. The reduced mass and highfrequency of operation of the nanomechanical resonant elements combinedwith their high Q values and power handling capabilities make the AlNNPR-S capable of achieving unprecedented values of limit of detectionand detection speed.

The performance of MN NPR-S in terms of sensitivity, limit of detectionand detection speed may be further improved by scaling thickness andincreasing the operating frequency of the AlN resonant nano plate whilemaintaining, at the same time, high values of Q and transductionefficiency. Such device scaling has previously been limited by thephysical and electrical properties of the metal electrode employed toprovide the excitation electrical signal to the piezoelectric nanoresonator. Ultra-thin metal electrodes, when scaled proportionally tothe AlN plate, may introduce high values of electrical resistance thatelectrically load the Q of the resonant element and limit the resolutionof the sensor. Thicker metal electrodes that are not scaledproportionally to the AlN plate may mechanically load the device byacting as a heavy mass on top of the nanoscale piezoelectric resonantplate, thereby negatively affecting both its Q and transductionefficiency.

A 2D graphene electrode may be located on top of an AlN resonant nanoplate. Graphene is a single-atomic-layer, 2D system composed solely ofcarbon atoms arranged in a hexagonal honeycomb lattice. This atomicallythick sheet of carbon is an excellent electrical conductor, withexceptionally high mobility for both electrons and holes of about 10⁵cm² V⁻¹ s⁻¹. The ultra-low mass 2D graphene layer may be employed, inlieu of a relatively thicker and heavier metal film, as a topelectrically floating electrode in a lateral field excitation schemeused to excite vibration in the piezoelectric nano plate. In at leastone instance, the 2D graphene top layer is the thinnest and lightestconductive electrode ever used to excite vibration in a piezoelectricNEMS resonator and has the potential to be used as an effectivechemically interactive material with the largest possible surface tovolume ratio. According to one embodiment, any suitable 2D electricallyconductive material may be employed as the electrode. According to oneembodiment, the electrode may be selected from at least one of grapheneand molybdenum disulfide. The electrode here may refer to the topelectrode, the bottom electrode, or both, depending on the application.

A three-dimensional schematic representation of the G-AlN NPR accordingto one embodiment is shown in FIG. 20. A high frequency bulk acousticmode of vibration may be excited into the AlN nano plate by aninterdigital bottom electrode 310 and an electrically floating topelectrode 312, which acts to confine the excitation field across thethickness of the piezoelectric layer 350. According to anotherembodiment, a metal-free NPR may be produced by utilizing a 2Delectrically active material for both/all electrodes contained withinthe NPR. In one embodiment, the top electrode, the bottom electrode, orboth, may comprise the 2D electrically active material. According to oneembodiment, the NPR may not include a metal electrode. According toanother embodiment, the NPR contains a metal electrode in addition tothe 2D electrically conductive electrode.

The application of an AC voltage to the interdigital electrode 310 mayexcite a contour-extensional mode of vibration through the equivalentd₃₁ piezoelectric coefficient of AlN. In the absence of the electricallyconductive top electrode, the excitation electric field may not beeffectively confined across the thickness, T, of the device, and theelectromechanical coupling coefficient, k_(t) ², of the nanomechanicalstructure may approach 0, and it would not be possible to excite thehigh frequency contour-extensional mode of vibration in such ultra-thinAlN nano-plate. Graphene has not previously been employed as anultra-thin and light top electrically floating electrode.

Given the equivalent mass density, ρ_(eq), and Young's modulus, E_(eq),of the material stack, including the AlN and electrodes, that forms theresonator, the center frequency, f₀, of this laterally vibratingmechanical structure, may be unequivocally set by the pitch, W₀, of theinterdigital bottom electrode. The resonance frequency of the device maybe expressed as

$f_{0} = {\frac{1}{2\; W_{0}}{\sqrt{\frac{E_{eq}}{\rho_{eq}}}.}}$

The other two geometrical dimensions, thickness, T, and length, L, mayhelp determine the equivalent electrical impedance of the resonator andmay be designed independently of the desired resonance frequency. Adevice with an effective device area of 55 μm (W)×196 μm (L), and thepitch 320, W₀, of bottom Platinum (Pt) finger electrode of the devicewas 20 μm, as shown in FIG. 21, resulting in a high ordercontour-extensional mode resonator working at high resonance frequencyof 245 MHz.

An important parameter for gravimetric sensing applications is theresonator sensitivity, S, to mass per unit area. For a NPR-S loaded onits top surface the sensitivity can be expressed as

$S = {- {\frac{f_{0}}{2\rho_{eq}T}.}}$

Therefore, the sensitivity, S, of a NPR gravimetric sensor may beimproved by increasing the operating frequency of the device andsimultaneously reducing its mass density and thickness. Such effectivedevice scaling may be achieved by replacing the heavy and thick topmetal electrode utilized in pre-existing sensors with a 2D and ultra-lowmass single atomic layer graphene electrode. According to oneembodiment, the graphene NPR may have a sensitivity of at least about 40kHz·μm²/fg—e.g., at least about 50 kHz·μm²/fg, about 55 kHz·μm²/fg,about 60 kHz·μm²/fg, about 65 kHz·μm²/fg, or more.

Table 1 reports the sensitivity, S, in units of kHz·μm²/fg, of the G-AlNNPR and the sensitivity of a pre-existing AlN NPR. The pre-existing AlNNPR was based on the same design as the G-AlN NPR but employed a 100 nmthick gold top electrode instead of the 2D graphene top electrode. Goldis a commonly employed top metal electrode in NEMS resonant sensorsbecause it may be easily functionalized with thiolated ligands. Asreported in Table 1, the G-AlN NPR-S exhibits more than a 2 foldimprovement in sensitivity as compared to the pre-existing AlN NPR.

TABLE 1 ρ_(eq) (kg · m⁻³) E_(eq) (GPa) T (μm) S AlN 6220 350 0.65 24.7AlN 7022 348 0.70 18.1 G-AlN 4192 396 0.55 53.1

The NPR may be fabricated by any suitable fabrication process. Accordingto one embodiment the graphene NPR may be fabricated by a combination oftop-down microfabrication techniques (6 masks) and bottom-up growth forgraphene. The top-down microfabrication techniques may be carried out ina wafer level process. The bottom-up growth of graphene may be carriedout in a die level process. According to one embodiment, the resonatormay be fabricated by a process including: disposing a first electrodeover a substrate, disposing a piezoelectric layer over the substrate andfirst electrode, disposing a second electrode including atwo-dimensional electrically conductive material over the piezoelectriclayer, etching the piezoelectric layer to form a piezoelectricnano-plate and releasing the nano-plate from the substrate. Thesubstrate may be any suitable material, such as silicon. According toone embodiment the substrate may be a silicon wafer. According to oneembodiment, the disposing of the piezoelectric layer may include anysuitable process, such as a sputter deposition process. Thepiezoelectric layer may include any suitable material, such as theaforedescribed piezoelectric materials. According to one embodiment, thedisposing of the first electrode may include any appropriate process,such as a sputter deposition process. The first electrode may be aninterdigital electrode. According to one embodiment, the disposing ofthe second electrode may include any appropriate process, such ascontacting a two-dimensional electrically active material with thepiezoelectric layer. According to one embodiment, the etching of thepiezoelectric layer to form a piezoelectric nano-plate may include anysuitable process, such as an ion conductive plasma (ICP) process. Theforming of the nano-plate may include forming a perimeter of thenano-plate. According to one embodiment, the releasing the piezoelectriclayer from the substrate may include any suitable process, such as anisotropic etching process.

The two-dimensional electrically conductive material may be formed byany suitable process. According to one embodiment, the two-dimensionalelectrically conductive material may be produced by a process including:forming the two-dimensional electrically conductive material on a foilsubstrate, disposing a polymer layer over the two-dimensionalelectrically conductive material, removing the foil substrate,contacting the two-dimensional electrically conductive material with thepiezoelectric layer and dissolving the polymer layer. The foil substratemay be any suitable foil, such as a metal foil, such as a Cu foil. Theforming of the two-dimensional electrically conductive material mayinclude any suitable process, such as chemical vapor deposition (CVD).The polymer layer may include any suitable polymer, such as poly-methylmethacrylate (PMMA). According to one embodiment, removing the foilsubstrate may include any suitable process, such as an etching process.The contacting the two-dimensional electrically conductive material withthe piezoelectric layer may include any suitable process, such asplacing the two-dimensional electrically conductive material on top ofthe piezoelectric layer. According to one embodiment, the dissolving thepolymer layer may include any suitable process, such as dissolving inacetone.

The fabrication process may additionally include a process to protectthe second electrode during the releasing process. According to oneembodiment, the additional process may include disposing a thin filmincluding a polymer over the second electrode prior to releasing thenano-plate and removing the thin film after releasing the nano-plate.The thin film may include any suitable polymer material, such aspolydimethyl glutarimide (PMGI).

An exemplary G-AlN NPR was fabricated as shown in FIGS. 29(a)-29(e). Ahigh resistivity (>10⁴Ω˜cm) silicon (Si) wafer was used as a substrate330. A 50 nm thick Platinum (Pt) film was sputter-deposited andpatterned by lift-off on top of the Si substrate to define the bottominterdigital electrode 310 as shown in FIG. 29(a). Then, a 500 nm AlNfilm 350 (stress 60 MPa and FWHM2.2°) was sputter-deposited and etchedby Inductively Coupled Plasma (ICP) etching in Cl₂ based chemistry todefine the perimeter of a resonant nano plate by forming gaps 360 in theMN film. Vias to access the bottom electrode were etched by H₃PO₄, asshown in FIG. 29(b). Then, a 100 nm thick gold (Au) film wassputter-deposited and patterned by lift-off to form probing pads 314that allow integration of the G-AlN NPR with electrical circuits, asshown in FIG. 29(c).

A macroscopic sheet of graphene was grown directly on a copper (Cu) foilby a chemical vapor deposition (CVD) method. The dimensions of thegraphene sheet were limited by the dimensions of the furnace employed inthe production of the sheet. The graphene sheet had external dimensionsof 1.5×1.5 cm. The graphene sheet was coated with a thin layer ofpoly-methyl methacrylate (PMMA) and then released from the Cu substrateby etching the Cu substrate with an aqueous iron (III) chloride (FeCl₃)solution. The graphene/PMMA sheet was rinsed in deionized (DI) water andplaced on top of the previously processed AlN NEMS die. The transfer ofthe graphene layer on the AlN NEMS die was completed by dissolving thePMMA in acetone. The graphene layer transferred on top of the AlN NEMSdie was then patterned by standard lithography and oxygen plasma etchingtechniques, to form a graphene top electrode as shown in FIG. 29(d). Toavoid unintentional doping of the graphene layer during the final NEMSresonator release step in XeF₂ a thin-film of polydimethyl glutarimide(PMGI) polymer was deposited by spin coating on top of the graphene-NEMSdie and patterned to protect the graphene electrodes. Finally, the G-AlNNEMS structure was released from the substrate by XeF₂ isotropic etchingof the silicon to form a release void 365 as shown in FIG. 29(e). ThePMGI protective layer was removed with a photoresist solvent.

High quality graphene was maintained throughout the fabrication processas confirmed by Raman spectrum taken after release of the G-AlN NPR, asshown in FIG. 22. The approximately 0.5 G-to-2D intensity ratio andsymmetric 2D band centered at about 2650 cm⁻¹ with a full width at halfmaximum of about 40 cm⁻¹ observed are typical features of monolayergraphene.

The fabricated G-AlN NPR was tested at room temperature and atmosphericpressure in an RF probe station and its electrical response was measuredby an Agilent E5071C network analyzer after performing a short-open-loadcalibration on a reference substrate. The electromechanical performanceof the device was extracted by Butterworth-Van Dyke (BVD) model fitting,as shown in FIG. 23, and compared to a pre-existing AlN NPR that wasfabricated on the same substrate and based on the same core design butemployed a 100 nm thick gold top electrode instead of the 2D graphenetop electrode. As shown in FIG. 24, the measured admittances of thefabricated G-AlN NPR exhibit a higher frequency response than thepre-existing AlN GPR.

Table 2 compares the BVD model fitting parameters of the fabricatedG-AlN NPR and the pre-existing AlN NPR. A higher operating frequency andunchanged Figure of Merit, k_(t) ²·Q, was achieved with the G-AlN NPR.Despite the ultra-reduced volume of the single atomic monolayer 2Dgraphene electrode, a relatively small and tolerable value of electricalresistance, R_(s), was introduced compared to the 150 times thicker and2500 times heavier gold electrode.

TABLE 2 f₀ Q k_(t) ² C₀ R_(m) R_(s) AIN 200 MHz 1015 1.81% 281 fF 189Ω 98Ω AIN 178 MHz  626 1.90% 324 fF 285Ω 128Ω G-AIN 245 MHz 1001 1.81%282 fF 157Ω 228Ω

Despite a mass reduced by about 43% and a volume reduced by about 16%,an increased sound velocity and a resonant frequency increased by about23%, the G-AlN NPR device exhibited an unchanged Figure of Merit ofabout 18 compared to the pre-existing AlN NPR device. This experimentalresult demonstrates that the introduction of the graphene electrode notonly enables the fabrication of AlN NPRs with lower volume and mass andimproved sensitivity to mass loading but also, despite the volumescaling, allows the achievement of high values of Q (˜1000), whichguarantee ultra-low noise performance of the sensor. Despite therelatively high sheet resistance of about 1.5 kΩ/□ of the synthesizedgraphene layer, the 2D graphene electrode introduced only a relativelysmall and tolerable value of electrical resistance, R₅, compared to a250 times thicker and 3750 times heavier gold electrode. It is worthnoting that a much lower sheet resistance of about 60Ω/□ can be achievedin commercially available graphene which would further reduce theelectrical loading of the graphene electrode to less than about 2% ofthe total loss of the system. The reduced mass and volume, and theincreased frequency of operation of such G-AlN NPRs combined with theirhigh Q values demonstrate the great potential of the proposed technologyfor the implementation of a new class of resonant sensors capable ofachieving unprecedented values of limit of detection and detectionspeed.

The fabricated G-AlN NPR and the pre-existing AlN NPR were directlywire-bonded to Pierce oscillator circuits implemented with an ATF-551M4E-pHEMT GaAs transistor. The resulting oscillator circuit 302 containingthe G-AlN NPR 300 is shown in FIG. 30. The phase noise was measured forbest bias conditions using an Agilent N9010A EXA signal analyzer.Despite the 250 fold smaller volume of the top electrode and the 38%higher operating frequency, improved phase noise performance wasrecorded for the G-AlN device due to its higher mechanical Q. Themeasured phase noise performance the oscillator circuits is shown inFIG. 25.

The electrical conductivity of the graphene electrode may be switched asthe result of the application of a DC bias or control signal. Accordingto one embodiment, the DC bias may be applied to a gate of theinterdigital electrode. The control signal may include a radio frequencysignal. According to one embodiment, the operation of the NPR mayinclude application of a DC bias or control signal to thetwo-dimensional electrically conductive material. After the applicationof a DC bias or control signal to the two-dimensional electricallyconductive material, the NPR may exhibit no mechanical resonance.According to one embodiment, after the application of a DC bias orcontrol signal to the two-dimensional electrically conductive material,the electromechanical coupling coefficient of the NPR may be about 0.The switching of the two-dimensional electrically conductive materialmay be reversible. According to one embodiment, the switching of theresonator may be reversed by applying at least one of a different DCbias and a different control signal to the two-dimensionallyelectrically active material of the resonator. According to oneembodiment, the switching of the resonator may not include the operationof a mechanical switch. The switching may produce the desired change inthe conductivity of the graphene electrode in a response time of about amicrosecond, or less.

FIG. 31(a) illustrates the operational mode of a NPR resonator includinga 2D electrically conductive electrode in a conductive state, accordingto one embodiment. The NPR may include a piezoelectric material 350,positive electrodes 345 and a negative electrode 340. The application ofa voltage across the positive electrodes 345 and the negative electrode340 may produce an electric field 315 within the piezoelectric material350, resulting in a piezo-induced strain 325 in the piezoelectricmaterial 350. FIG. 31(b) illustrates a similar NPR resonator including a2D electrically conductive electrode in a non-conductive, insulating,state, according to one embodiment. The insulating electrode 395 may notproduce an electric field 315 within the piezoelectric material 350 uponapplication of a voltage across the positive electrodes 345 and thenegative electrode 340 that is capable of producing a piezoelectricstrain, and thus the NPR with an electrode in an insulating state maynot produce the resonant behavior exhibit by a NPR with an electrode ina conductive state.

In one example, a graphene electrode containing MN NPR was fluorinatedwith xenon difluoride (XeF₂) gas to increase the resistivity of thegraphene layer by doping. Doping of a graphene layer may produce anincrease in resistivity of up to 6 times in comparison to a non-dopedgraphene layer. A comparison of the operation of the undoped G-AlN NPRand the fluorinated G-AlN NPR is shown in FIG. 26. Additionally, thetuning of graphene conductivity as a result of the application of a DCcontrol bias has been demonstrated. As shown in FIGS. 27 and 28, theapplication of a DC control bias to the graphene electrode changes theresistivity of the graphene electrode and alters the resonance behaviorof the G-AlN NPR. The application of a DC bias may produce up to a 30fold increase or decrease of the electrical conductivity of a grapheneelectrode. The switching effect produced by the application of a DCcontrol bias may be reversible.

Tuning the electrical conductivity of the 2D electrically conductiveelectrode material allows the control of the electromechanical couplingof the G-AlN NPR and implementation of banks of multi frequencyresonators and filters in electrically programmable matrices that do notrequire mechanical switches in the RF line. This out-of-line switchingcapability may reduce the capacitive loading of turned off resonators orfilters in the bank. According to one embodiment, the switching of theresonator may allow the input/output capacitance of the resonator to bereduced and/or eliminated when not in use. The input/output capacitancemay be reduced by more than 3 orders of magnitude. In effect, theresonators may be switched on and off by controlling theelectromechanical coupling factor. This allows individual resonators ina filter bank to switched on and off by individually controlling theelectromechanical coupling factor. By introducing an electrode withtunable conductivity, such as graphene, it is possible to develop a newclass of MEMS resonators in which the electromechanical couplingcoefficient, k_(t) ² can be switched on and off by applying a DC bias ora control signal. According to one embodiment, the conductivity of theelectrode may be tuned to create an effective open circuit within theresonator. Additionally, the out-of-line switching enabled by theswitching processes described herein reduces resistive losses in the RFsignal line, producing improved receiver performance.

ADDITIONAL NOTES

The various embodiments herein may be combined. For example, a radiationdetector may include a resonator with a 2D electrically conductivematerial or combined mode resonator.

All literature and similar material cited in this application,including, but not limited to, patents, patent applications, articles,books, treatises, and web pages, regardless of the format of suchliterature and similar materials, are expressly incorporated byreference in their entirety. In the event that one or more of theincorporated literature and similar materials differs from orcontradicts this application, including but not limited to definedterms, term usage, described techniques, or the like, this applicationcontrols.

While the present teachings have been described in conjunction withvarious embodiments and examples, it is not intended that the presentteachings be limited to such embodiments or examples. On the contrary,the present teachings encompass various alternatives, modifications, andequivalents, as will be appreciated by those of skill in the art.

While various inventive embodiments have been described and illustratedherein, those of ordinary skill in the art will readily envision avariety of other means and/or structures for performing the functionand/or obtaining the results and/or one or more of the advantagesdescribed herein, and each of such variations and/or modifications isdeemed to be within the scope of the inventive embodiments describedherein. More generally, those skilled in the art will readily appreciatethat all parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the inventive teachingsis/are used. Those skilled in the art will recognize many equivalents tothe specific inventive embodiments described herein. It is, therefore,to be understood that the foregoing embodiments are presented by way ofexample only and that, within the scope of the appended claims andequivalents thereto, inventive embodiments may be practiced otherwisethan as specifically described and claimed. Inventive embodiments of thepresent disclosure are directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe inventive scope of the present disclosure.

All definitions, as defined and used herein, should be understood tocontrol over dictionary definitions, definitions in documentsincorporated by reference, and/or ordinary meanings of the definedterms.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.” Any ranges citedherein are inclusive.

The terms “substantially” and “about” used throughout this Specificationare used to describe and account for small fluctuations. For example,they may refer to less than or equal to ±5%, such as less than or equalto ±2%, such as less than or equal to ±1%, such as less than or equal to±0.5%, such as less than or equal to ±0.2%, such as less than or equalto ±0.1%, such as less than or equal to ±0.05%.

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Multiple elements listed with“and/or” should be construed in the same fashion, i.e., “one or more” ofthe elements so conjoined. Other elements may optionally be presentother than the elements specifically identified by the “and/or” clause,whether related or unrelated to those elements specifically identified.Thus, as a non-limiting example, a reference to “A and/or B”, when usedin conjunction with open-ended language such as “comprising” may refer,according to one embodiment, to A only (optionally including elementsother than B); in another embodiment, to B only (optionally includingelements other than A); in yet another embodiment, to both A and B(optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element of a number or list of elements. In general, the term “or”as used herein shall only be interpreted as indicating exclusivealternatives (i.e. “one or the other but not both”) when preceded byterms of exclusivity, such as “either,” “one of,” “only one of,” or“exactly one of” “Consisting essentially of,” when used in the claims,shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) mayrefer, according to one embodiment, to at least one, optionallyincluding more than one, A, with no B present (and optionally includingelements other than B); in another embodiment, to at least one,optionally including more than one, B, with no A present (and optionallyincluding elements other than A); in yet another embodiment, to at leastone, optionally including more than one, A, and at least one, optionallyincluding more than one, B (and optionally including other elements);etc.

As used herein “at %” refers to atomic percent and “wt %” refers toweight percent. However, in certain embodiments when “at %” is utilizedthe values described may also describe “wt %.” For example, if “20 at %”is described according to one embodiment, in other embodiments the samedescription may refer to “20 wt %.” As a result, all “at %” valuesshould be understood to also refer to “wt %” in some instances, and all“wt %” values should be understood to refer to “at %” in some instances.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” “composed of,” and the like are tobe understood to be open-ended, i.e., to mean including but not limitedto. Only the transitional phrases “consisting of” and “consistingessentially of” shall be closed or semi-closed transitional phrases,respectively, as set forth in the United States Patent Office Manual ofPatent Examining Procedures, Section 2111.03.

The claims should not be read as limited to the described order orelements unless stated to that effect. It should be understood thatvarious changes in form and detail may be made by one of ordinary skillin the art without departing from the spirit and scope of the appendedclaims. All embodiments that come within the spirit and scope of thefollowing claims and equivalents thereto are claimed.

What is claimed is:
 1. A radiation detector comprising: a resonatorcomprising: a piezoelectric plate, and an interdigital electrodedisposed on a first surface of the piezoelectric plate, and an absorberlayer disposed above a second surface of the piezoelectric plateopposing the first surface, wherein the absorber layer is capable ofabsorbing at least one of infrared and terahertz radiation.
 2. Thedetector of claim 1, wherein the detector is capable of room temperatureoperation.
 3. The detector of claim 1, wherein a contour-mode ofvibration is excited in the piezoelectric plate in response to analternating current (AC) applied through the electrode.
 4. The detectorof claim 1, wherein the absorber layer comprises at least one of asingle wall carbon nanotube forest, silicon nitride, graphene, andphotonic meta-materials.
 5. The detector of claim 1, wherein atemperature coefficient of frequency of the resonator is less than about−30 ppm/K.
 6. The detector of claim 1, wherein the absorber layercontacts the second surface of the piezoelectric plate.
 7. The detectorof claim 1, wherein the absorber layer and the second surface of thepiezoelectric late are separated by an air gap.
 8. The detector of claim1, further comprising a complementary metal-oxide-semiconductor circuitconfigured to provide an electronic readout of a resonance frequency ofthe resonator.
 9. The detector of claim 1, wherein the piezoelectricplate comprises at least one of aluminum nitride, lithium niobate,lithium tantalite, zinc oxide, gallium nitride and quartz.